Add GUI desktop application

- Add gui/ package: CustomTkinter app with dashboard, mods, tools, logs,
  and settings views; first-run SetupWizard for config.json generation
- Add gui.py root entry point (calls gui.run_app())
- Add selection.json for GUI selection state persistence
- Add customtkinter to requirements.txt
- Fix link_mods.py minor issues surfaced during GUI integration
- Add modlist_html/Test_Preset_A.html and Test_Preset_B.html as example inputs
- Update README.md: add GUI prerequisites, gui.py script section, gui/ folder
  structure, customtkinter to prerequisites table
- Update CLAUDE.md: add python gui.py to common commands, document GUI package
  architecture and views

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

gui/_constants.py

@@ -0,0 +1,28 @@
+from __future__ import annotations
+
+from pathlib import Path
+
+# ---------------------------------------------------------------------------
+# Window geometry
+# ---------------------------------------------------------------------------
+
+SIDEBAR_W: int = 190
+APP_TITLE: str = "Arma Mod Manager"
+
+# ---------------------------------------------------------------------------
+# Status colours
+# ---------------------------------------------------------------------------
+
+COLOR_OK:      str = "#4CAF50"
+COLOR_PENDING: str = "#9E9E9E"
+COLOR_RUNNING: str = "#2196F3"
+COLOR_ERROR:   str = "#F44336"
+COLOR_WARN:    str = "#FF9800"
+
+# ---------------------------------------------------------------------------
+# Filesystem
+# ---------------------------------------------------------------------------
+
+# gui/ lives one level below the project root
+PROJECT_ROOT:   Path = Path(__file__).parent.parent
+SELECTION_FILE: Path = PROJECT_ROOT / "selection.json"